1. Field of the Invention
The present invention relates to polymeric membranes which are characterized by high salt rejection and high urea rejection properties. More particularly, the present invention relates to membranes prepared by the deposition of a polymeric material from an RF plasma with the simultaneous incorporation of nitrogen within the deposited membrane to form a polymeric membrane of high urea rejection properties.
2. DESCRIPTION OF THE PRIOR ART
Advances in semipermeable membrane technology have led to the development of many types of membranes useful in reverse osmosis techniques for the purification and clarification of aqueous solutions. Many membranes are known which perform effectively as salt rejecting membranes in the purification of brine solutions by reverse osmosis techniques. Moreover, in some special applications such as the treatment of human fluid wastes in prolonged manned space missions, it is especially imperative that all waste water be purified to a level suitable for human consumption. This means that any potentially useful membrane for such applications should not only be capable of salt rejection, but also should be capable of rejecting urea if a portable water filtered product is to be obtained.
Various techniques have been employed in the past for the manufacture of semipermeable membranes which include various casting methods such as spin casting, dip casting, doctor blade casting, and the like as well as other conventional techniques. The membranes have been formed into various shapes such as sheets, tubes, fibers and the like. In still another technique as disclosed in U.S. Pat. No. 3,847,652 an RF plasma has been used to form a polymeric film which is useful as a salt rejecting reverse osmosis membrance. In the method a liquid, ethyleneically unsaturated compound such as allyl amine is subjected to an RF field on the order of 30 to 50 watts in a vacuum of generally less than 0.3 torr. The deposition of the polymeric membrance can be accomplished in the presence of an additive gas such as nitrogen, helium, argon or the like, which generally increases the rate of polymerization. The resulting membrane, while possessing good salt rejecting properties, is not very effective in rejecting urea from aqueous waste solutions. In fact, its urea rejecting abilities are comparable to those of the commercially produced and widely used cellulose acetate membranes which exhibit less than 20% urea rejection.
In another prior art technique as shown in U.S. Pat. No. 3,775,308, a polymeric film is formed by subjecting such liquid vinyl monomers as vinylpyridine, vinylpyrrolidine or the like to electrodeless glow discharge polymerization. The polymeric membrane is deposited on a porous substrate of a material such as cellulose acetate, polysulfone, cellulose butyrate, or the like. However, the membranes of the reference also lack sufficient urea rejection properties. Furthermore, the polymerizable vinylmonomers are liquid and consequently it is difficult to control the addition of the monomer to the reaction chamber.
A need, therefore, continues to exist for a polymeric membrane which when used in the reverse osmosis purification of aqueous solutions, exhibits high urea rejecting properties.